The present invention relates to a power amplification circuit using semiconductor devices and, in particular to a semiconductor power amplification circuit equipped with an output short-circuit protection circuit to protect a transistor which is a part of the circuit, from being destroyed when the output terminal is short-circuited with one terminal of the power supply means.
The conventional transistorized power amplification circuit has a circuit construction of the output stage in which an output transistor operating as an NPN-type transistor and an output transistor as a PNP-type transistor are connected in series between the power supply lines, and the common connecting point of these two transistors is used as an output terminal, and in which a series circuit of a large capacitance capacitor and a loud speaker is connected between the above output terminal and one of the power supply line. However, such a basic circuit configuration of a power amplification circuit output stage alone has a drawback that, when the output terminal is connected directly to the one of the power supply lines to form a short circuit, an unlimited current flows through the output transistor positioned opposite to the power supply line connected to the output terminal, resulting in a breakdown of this output transistor.
As conventional means for overcoming such a drawback, there has been proposed a protection circuit in which a resistor having a resistance of 0.5 to several ohms is inserted between the output electrode of the output transistor such as the emitter and the output terminal, and, by detecting the voltage drop across the above resistor, the base and the emitter electrodes of the transistor through which large current is flowing are short-circuited to cause it to be in a cut-off condition, and thereby the breakdown of the output transistor is prevented. Such a protective circuiit has another shortcoming that of an output power loss due to the presence of the resistor positioned between the output electrode of the output transistor and the output terminal.